For example, in an optical transmitter and receiver provided in an optical transmission system transmittable a high-speed (broadband) optical signal of 40 Gb/s or the like, an electric signal for driving an optical modulator or the like which is provided in the optical transmitter and receiver is a high frequency signal in accordance with the high-speed optical signal. In some of devices processing such a high frequency signal, it is necessary to perform electric connection between the devices by blocking a DC (direct current) component while coupling AC (alternate current) components. In recent years, in response to a demand for device miniaturization, a DC block circuit for blocking the DC component tends to be incorporated in a package of the device.
The DC block (or DC cut) circuit incorporated in the package is provided as a substrate for high frequency on which a transmission line for transmitting and receiving the high frequency signal between an IC chip in the package and a circuit or the like outside the package is formed, and a chip condenser (chip capacitor) or the like is mounted on the transmission line. The transmission line is formed by patterning on a front surface of a dielectric substrate such as ceramics, and a grounded conductor is formed on a rear surface of the dielectric substrate, so that a microstrip line configuration is provided as one example. In the way of the transmission line formed on the front surface of the dielectric substrate, two land portions wider than the other portion opposite to each other with a gap therebetween are formed, and an electric component, such as the chip condenser, is mounted on the land portions so as to cross the gap. The grounded conductor formed on the rear surface of the dielectric substrate is in contact with a metal substrate provided as a housing of the package, for example. Namely, the dielectric substrate is disposed on the metal substrate in the state where the grounded conductor is in contact with the metal substrate.
The line width and the like of the transmission line for high frequency formed in the DC block circuit is designed in order to match with a predetermined characteristic impedance such as 50Ω. In recent years, since a bonding pad of the IC chip connected to the transmission line becomes narrow pitched, the transmission line is designed so that the predetermined characteristic impedance is obtained in the narrow line width corresponding to the narrow pitch. However, since the chip condenser or the like needs to be mounted on a DC blocking portion (the gap and the land portions) of the transmission line, the line width of the DC blocking portion needs to be widen for using as a connecting land corresponding to the chip condenser or the like. Accordingly, it is necessary to consider how a characteristic impedance of the land portions in which the line width is made wider, is made to match with the predetermined characteristic impedance such as 50Ω at the other portion than the land portions, and therefore, such a technique has been disclosed in Japanese Laid-open Patent Publication No. 09-107210 (to be referred to as “Patent Documents 1”) and Japanese Laid-open Patent Publication No. 2008-125038 (to be referred to as “Patent Document 2”).
According to the technique disclosed in each of Patent Documents 1 and 2, as to the land portions of the transmission line, line widths of which are made wide for mounting the electric component, the grounded conductor in a region where such land portions are projected onto the rear surface of the dielectric substrate, is eliminated from the dielectric substrate, so that an opening portion is formed on the land portions-corresponding region in the grounded conductor. Further, a concave portion (dug groove) is formed on the metal substrate (described as a metallic case or a metal base in Patent Documents) below the opening portion in the grounded conductor, and the depth of the concave portion is designed at a specific value, so that the characteristic impedance of the land portions of the transmission line positioned above the concave portion is matched with a target value.
As described above, in the structure in which the concave portion is disposed on the metal substrate so that a capacity to the ground of the land portions of the transmission line is regulated, the characteristic impedance is determined depending on the depth of the concave portion (refer to “h2”, “D” described in Patent Documents), and thus, the concave portion needs to be processed with precision. However, since the processing of the concave portion on the metal substrate needs to be performed by mechanical process, the processing precision is not so high (tolerance of about ±50 μm). Accordingly, the technique disclosed in Patent Documents is not advantageous for the high frequency to be used in optical transmission speed of 40 Gb/s or higher. Further, if the position at which the concave portion is formed and the position of the opening portion in the grounded conductor do not accurately correspond to each other, the characteristic impedance is affected. Therefore, there is a problem to be solved in that the positioning of the opening portion relative to the concave portion needs to be performed with precision, namely, the disposing position of the dielectric substrate on the metal substrate needs to be performed with precision.
Focusing on the above-mentioned problem, the above substrate for high frequency needs to have a structure which makes the characteristic impedance matching much easier and is further suitable for high frequency than the conventional structure.